The present invention relates to an image forming apparatus applicable to a laser printer, facsimile apparatus or similar digital image forming apparatus or a display. More particularly, the present invention relates to an electrophotographic color copier, color printer or similar color image forming apparatus for stably starting reproducing the highlight portion of an image from low density and enhancing the resolution and stability of a medium to a high density portion.
Generally, an electrophotographic image forming apparatus scans a photoconductive element with a laser beam for forming a latent image on the element and develops the latent image with toner. This kind of apparatus is implemented as, e.g., a digital copier, a laser printer or a facsimile apparatus. Today, an electrophotographic copier, printer or similar color image forming apparatus is also available for forming a full-color image. It is necessary with a color image forming apparatus to improve the reproducibility of dots and line dots in a low density portion and to enhance the stability of tonality and color reproducibility against environment.
Japanese Patent Laid-Open Publication Nos. 7-254985, 7-254988, 7-283942, 8-114956 and 8-125863, for example, disclose various technologies relating to halftone processing essential with a color image forming apparatus and mainly directed toward the reproducibility of a highlight portion. Specifically, the technologies taught in the these documents each use PWM (Pulse Width Modulation) for a writing purpose and execute dither processing weighted by two dots in the main scanning direction, thereby stably reproducing a highlight portion with a small number of lines. In addition, the above technologies define a beam diameter and a pixel distance in the main scanning direction for the stable reproduction of a highlight portion. However, the dither processing fails to faithfully reproduce density in a fine area and therefore causes image data to be lost, e.g., causes a line to disappear and brings about a color moiré problem.
Japanese Patent Laid-Open Publication Nos. 4-200075, 4-200076, 4-200077, 4-200078, 5-284339, 5-292302 and 6-62248, for example, teach technologies for reducing banding and image noise and thereby stabilizing image density by use of a two-dot multi level system. The two-dot multilevel system is the combination of multilevel writing using one-dot modulation and a fine matrix involving a minimum of fall of resolution. These technologies are applicable to, e.g., a digital copier and characterized by adding the density data of two nearby dots and distributing the resulting sum. More specifically, a matrix having two dots in the main and/or the horizontal direction is combined with one-dot, 256 tonality derived from the multilevel modulation of a semiconductor laser. This is successful to further enhance the reproducibility of a halftone density range. The present invention constitutes an improvement over this kind of scheme.